The present invention relates to a constant-velocity universal joint.
The invention applies in particular to tripot constant-velocity joints for motor vehicle transmissions.
A tripot constant-velocity joint of this kind generally comprises a male element with ternary symmetry, or tripod, secured to a first rotating shaft, and a female element with ternary symmetry, or bell housing, secured to a second rotating shaft.
In an embodiment known from document FR-A-2 701 741, each mechanical transmission member comprises an internal ring, arranged inside the external roller, and which swivels and slides about a spherical bearing surface of the corresponding arm.
Each mechanical transmission member also comprises means of coupling the internal ring and the external roller to allow their relative pivoting about a common axis of revolution. The coupling means allow only a limited relative translational movement between the internal ring and the external roller along the common axis of revolution.
In general, the external roller comprises a peripheral surface of torus-shaped cross section. The, transverse profile of each track is in the form of a broken arc which is symmetric with respect to a mid-plane orthogonal to the corresponding longitudinal and radial plane.
Such a structure, through the rolling of the external rollers along one of the tracks of the corresponding pair, allows the constant-velocity joint to operate at an angle of misalignment between the two rotating shafts when a driving torque is applied.
During such operation under torque, each peripheral surface of an external roller bears against a track of the corresponding pair of tracks, and there is a small clearance between this peripheral surface and the other track of said pair. In addition, each arm is given a back and forth translational movement with respect to the corresponding pair of tracks parallel to the corresponding longitudinal and radial plane. This back and forth movement in the longitudinal and radial plane is due, on the one hand, to the inclination of the arm and, on the other hand, to the orbital movement, known as the offset, of the tripod at a frequency which is three times the rotational speed, as is well known in the art.
Such a back and forth movement of the arms gives rise, in the case of each external roller, to a back and forth rocking movement of the roller about the part of its peripheral surface bearing against one of the tracks. The rocking movement is brought about on the one hand by the friction between the spherical bearing surface of the arm and the corresponding internal ring and, on the other hand, by the moving of the point of contact between the bearing surface of the arm and the corresponding internal ring.
Thus, for each arm, the part of the peripheral surface of the roller which is diametrically opposite the part which is bearing oscillates between the two half-arcs forming the broken-arc profile of the track on which the peripheral surface is not bearing.
Such an oscillatory movement gives rise to friction phenomena and may even cause jamming between the roller and the track on which it is not bearing, particularly in the half-arc of the track profile located farthest toward the inside of the bell housing.
The purpose of the invention is to overcome these problems by providing a mechanical joint which can operate with an angle of misalignment with less friction and at the same time limiting the risk of jamming.
Another subject of the invention is a mechanical transmission member for a universal joint as defined hereinabove.